1. Field of the Invention
This invention relates to a heat developable light-sensitive material and particularly to a heat developable light-sensitive material having on the back surface of the support thereof one layer containing a compound with a ##STR2## unit to improve the retention of the freshly manufactured properties of the light-sensitive material (hereafter the freshness retention capability), especially in those situations where the materials are stacked in a superposed condition upon other such materials.
Freshness-retention capability means the capability of light-sensitive materials to retain their fresh photographic characteristics during storage extended periods. This is an extremely important commercial property, as the photographic characteristics of light-sensitive materials poor in freshness-retention capability change from the time of manufacture until they come into consumers' hands, which lowers the commercial value thereof. Moreover, it is undesirable for consumers that the characteristics of light-sensitive materials at the completion of their use are different from their characteristics at the beginning of use.
2. Description of the Prior Art
Silver halide-using photography has been much more universally carried out, as compared to electron photography, diazo photography and the like, as the former offers excellent photographic characteristics such as sensitivity, gradation, etc., as compared to the latter. However, silver halide photography requires extensive time and labor as silver halide light-sensitive materials must be development-processed with a developing solution after image-wise exposure, and then subjected to several treatments such as stopping, fixing, washing, stabilizing and other similar processings, using solutions to prevent a developed image from changing color or deteriorating under indoor light, or in order to protect undeveloped white areas (called background) from blackening. In addition, there and other troubles with the use of conventional silver halide materials such as danger to humans due to the chemical agents used in the above method, e.g., processing areas and workers' hands and clothes are often contaminated with the reagents used. Further, pollution is often caused by discarding the solutions used in the above-mentioned treatments into rivers.
Therefore, it has been strongly desired to provide highly sensitive silver halide photographic materials which can be dry processed instead of processed in solutions, so as to provide stable developed images and to reduce background color changes under exposure to indoor light.
Much effort has been expended the art to solve these problems. For example, the incorporation of a 3-pyrazolidone developing agent into a silver halide emulsion made it possible to develop light-sensitive materials by the application of heat, as disclosed in German OLS Nos. 1,123,203, 1,174,157 and so on. In addition, developing speed was accelerated by adding a water-donating substance to the above-mentioned developing agent when the above-mentioned emulsion system is heated, as disclosed in German OLS No. 1,175,075. Moreover, the addition of a fixing agent for silver halide to the above-mentioned developing agent is described in German OLS No. 1,003,578. However, these techniques do not completely stabilize the silver halide particles which remain in the dry processed light-sensitive material to light, i.e., dry fixing is not described in the first three patents above and, as one would expect, the presence of a developing agent (reducing agent) and a fixing agent as described in the last patent above causes undesirable reactions on storage, and renders the last method above impractical for commercial use.
In the art of light-sensitive materials which can provide photographic images by receiving photographic treatments in the dry condition, the most successful ones are, at present, the heat developable light-sensitive materials which are disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075, wherein the compositions contain as essential components silver salts of organic acids, a small amount of silver halide and a reducing agent. In such a light-sensitive system, silver halide particles remain in the light-sensitive material after developing without stabilizing. Nevertheless, such light-sensitive systems provide satisfactory results, as do light-sensitive materials which contain residual silver halide which has been stabilized to light. This is because the slight coloration resulting from the decomposition of such a small amount of residual silver halide caused by light-exposure can barely be perceived by human eyes since only a small amount of silver halide is used, and a large portion of the silver component exists in the form of white or pale yellow organic silver salts which are so stable to light that they hardly blacken upon light-exposure. Such light-sensitive systems an stable at ordinary temperatures, but when heated, usually up to 80.degree. C., preferably 100.degree. C., after image-wise exposure, they form an images because an oxidizing agent (organic silver salt) and a reducing agent present in a light-sensitive layer thereof undergo a redox reaction in the presence of a catalytic amount of exposed silver halide in the vicinity thereof to liberate silver which quickly blackens exposed areas of the light-sensitive layer and causes contrast between exposed areas and unexposed areas (background).
The present invention relates to an improvement on the above-described heat developable light-sensitive materials.
As described, the three essential components of such heat developable light-sensitive materials are an organic silver salt, a small amount of silver halide and a reducing agent. Usually, the light-sensitive materials containing these three components only provide yellow or pale yellow images when heat-developed after the image-wise exposure. Therefore, color toning agents are necessary so that dark tone images, i.e., black, brownish black, greenish black or violet-black images, can be obtained. Thus far, a wide variety of compounds are known as such color toning agents.
The effectiveness of a color toning agent is influenced by the organic silver salt and the reducing agent used in combination therewith. Most commonly used color toning agents include heterocyclic organic compounds which contain at least two hetero atoms, and which contain at least one nitrogen atom in the heterocyclic ring thereof, as disclosed in U.S. Pat. No. 3,080,254. Specific examples thereof include phthalazone (phthalazinone), phthalic anhydride, 2-acetylphthalazinone, 2-phthalylphthalizinone, substituted phthalazinones as disclosed in U.S. Pat. No. 4,076,534, pyrazoline-5-ones as disclosed in Japanese Patent Laid-Open No. 6077/71; cyclic imides such as phthalimide, N-hydroxyphthalimide, N-potassium phthalimide, phthalimide silver, etc.; quinazolines; phthalazinone silver; mercapto compounds as disclosed in Japanese Patents (Laid-Open) 5019/74 and 5020/74; oxazinediones as disclosed in U.S. Pat. No. 3,885,967; phthalazinediones as disclosed in U.S. Pat. No. 4,076,534; uracyls as disclosed in Japanese Patent Application No. 18378/74; N-hydroxy-naphthalimides as disclosed in U.S. Pat. No. 3,782,941; substituted phthalimides as disclosed in German OLS Nos. 2,140,406, 2,141,063 and 2,220,597, and phthalazinone derivatives as disclosed in German OLS No. 2,220,618.
However, only phthalazinone is accepted as an excellent color toning agent which enables the formation of black tone images, and which does not impair desireable photographic properties, e.g., anti-thermal fogging capability, light-stability, sensitivity, density and the like. Of course, the combined use of phthalazinone and the above-mentioned other color toning agents can provide considerably good results. Phthalazinone is effective when used in concentrations ranging from about 0.01 mole to about 5 mole, preferably from about 0.1 mole to about 2 mole, per mole of organic silver salt.
However, phthalazinone-containing heat developable light-sensitive materials suffer one serious disadvantage: they are poor in freshness retention capability, that is, the photographic characteristics of such light-sensitive materials are not stable upon storage. Long-term storage of such light-sensitive materials under high temperature and high humidity remarkably lowers their density and sensitivity to result in the disappearance of images. It is thought that one of the reasons for the decrease in density and sensitivity is that phthalazinone in the light-sensitive material sublimes into the air on a storage.
More particulary, such a light-sensitive material shows poor preservability when the light-sensitive layer thereof is in contact with the back surface of a support (e.g., it is rolled, or it is cut into sheets which are piled up.). This is a serious disadvantage, for light-sensitive materials usually come into the consumers' hands in such a state that a light-sensitive layer is in contact with the back surface of a support.
Freshness-retention capability can be rapidly examined by the use of a forced deterioration test on the light-sensitive materials, where the light-sensitive materials are exposed to air at elevated temperatures. It has been found that a forced deterioration test at 50.degree. C. for 24 hours corresponds approximately to the deterioration over three months at room temperature (20.degree. C.). Therefore, the freshness-retention capability at room temperature can be predicted from this forced deterioration test.
The solution of the above-mentioned problems has been an important goal in this art.